Projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction.
In a common operating mode, multimedia projection systems receive analog video signals from a personal computer ("PC"). The video signals may represent still, partial-, or full-motion display images of a type rendered by the PC. The analog video signals are typically converted in the projection system into digital video signals, and the signals are electronically conditioned and processed to control an image-forming device, such as a liquid crystal display ("LCD") or a digital micromirror device ("DMD").
A popular type of multimedia projection system employs a broad spectrum light source and optical path components upstream and downstream of the image-forming device to project the image onto a display screen. An example of a DMD-base multimedia projector is the model LP420 manufactured by In Focus Systems, Inc., of Wilsonville, Ore., the assignee of this application.
Significant effort has been invested into developing projectors producing bright, high-quality, color images. However, the optical performance of conventional projectors is often less than satisfactory. For example, suitable projected image brightness is difficult to achieve, especially when using compact portable color projectors in a well-lighted room. The projectors typically use high intensity arc lamps as their light source and then filter out all of the light except for blue, green, and red light and employ three separate light paths or some forms of sequential color modulator to coordinate color image data.
Because LCD displays have significant light attenuation and triple path color light paths are heavy and bulky, portable multimedia projectors typically employ DMD displays in a single light path configuration. Producing a projected color image with this configuration typically requires projecting a frame sequential image through a sequential color modulator, such as a color wheel.
FIG. 1 shows a typical prior art frame sequential color (FSC) display system 10 in which a sensor 12 senses a timing mark 14 to detect a predetermined color index position of a motor 16 that rotates a color wheel 18 having respective red, green, and blue filter segments R, G, and B. A light source 20 projects a light beam 22 through color wheel 18 and a relay lens 24 onto a display device 26, such as an LCD-based light valve or a DMD. A display controller (not shown) drives display device 26 with sequential red, green, and blue image data that are timed to coincide with the propagation of light beam 22 through the respective filter segments R, G, and B of color wheel 18. A DC motor rotates color wheel 18 at about 6,650 rpm to about 7,500 rpm. Clearly, successful operation of a FSC display system depends on properly synchronizing the red, green, and blue image data to the angular position of color wheel 18.
Sensor 12 typically employs any of optoelectrical or electromechanical shaft position or motor armature position detectors and usually requires some means for aligning timing mark 14 to the start of one of the filter segments. This alignment is typically a costly and error prone mechanical adjustment that accounts for angular differences between motor 16 and the mechanical mounting of filter segments R, G, and B. Of course, electrical or mechanical delays associated with sensor 12 further contribute to alignment errors.
The accumulated angular errors open the possibility of synchronization errors between the red, green, and blue image data to the angular position of color wheel 18, a possibility that prior workers avoided by building a timing duty cycle into the display controller electronics. The timing duty cycle provides for driving display device 26 with the red, green, and blue image data for only a portion of the time when light beam 22 is propagating through each of respective filter segments R, G, and B to prevent presenting display device 26 with an improper color. Unfortunately, the timing duty cycle reduces the total amount of illumination available for displaying each color and, therefore, reduces the brightness of the resultant displayed color image. In addition, color wheels 18 and their associated motors are heavy and noisy.
An alternative technique that substantially eliminates any mechanical, optical, and electrical rotational timing errors that are intrinsic to color wheel systems is, therefore, desired.